A) Field of the Invention
The present invention relates to a device having a capacitor and its manufacture method, and more particularly to a device having a ferroelectric capacitor and its manufacture method.
B) Description of the Related Art
If a ferroelectric layer is used as the dielectric layer of a capacitor, polarization in the ferroelectric layer can be maintained even after voltage applied to the capacitor is removed. A nonvolatile memory can therefore be formed by a combination of ferroelectric capacitors and transistors for selectively applying voltage to each capacitor. Assuming that the same voltage is applied across opposing electrodes having a unit area, the larger the polarization in the ferroelectric layer, a nonvolatile memory having better retention characteristics can be obtained. An orientated ferroelectric layer has a larger polarization amount than a non-orientated ferroelectric layer so that the orientated ferroelectric layer is effective for improving the retention characteristics.
The surface of a transistor formed on a semiconductor substrate is covered with an insulating film made of silicon oxide or the like. The insulating film is generally of an amorphous phase or substantially an amorphous phase. A metal film such as a wiring layer formed on the insulating film takes a polycrystalline phase. A ferroelectric layer formed on the non-orientated surface of an amorphous phase or a polycrystalline phase will not be orientated.
Reports such as Journal of Applied Physics, 1991, vol. 70, no. 1, pp. 382 to 388 indicate that a platinum or iridium layer can be made to have a (1 1 1) orientation by heating it, even if the underlying layer is amorphous or polycrystalline. A ferroelectric layer formed on a (1 1 1) orientated metal layer can be made to have a (1 1 1) orientation. For example, there are experiments that a PbZrxTi1-xO3 (PZT) film is formed on an underlying electrode of platinum or the like and made to have a (1 1 1) orientation.
It is known that if opposing electrodes sandwiching a ferroelectric layer of PZT or the like are made of noble metal such as platinum, the ferroelectric layer undergoes fatigue with time. It is also known that fatigue of a ferroelectric layer can be prevented if it is sandwiched between conductive metal oxide. In this context, there are experiments that after an orientation film of noble metal such as platinum is formed, a conductive metal oxide film is formed orientated in conformity with the orientation of the underlying surface, and on this conductive metal oxide film, an orientated ferroelectric film is formed.
The structure of such a ferroelectric capacitor is proposed in JP-A-HEI-9-245525. LaNiO3 is disclosed as an example of the material of a conductive metal oxide film.
PZT and the like are known as the material of a ferroelectric layer having the chemical formula of an ABO3 and a pervskite crystal structure.
FIG. 5A is a schematic diagram showing an ABO3 type pervskite crystal structure. A unit cell is approximately represented by a cube. In FIG. 5A, an atom A represented by a circle is disposed on each corner of the cube. An atom B represented by a square is disposed at the body center of the cube. An atom O represented by a triangle is disposed at the center of each plane.
PbZrxTi1-xO3 is tetragonal in the range of 0≦x<0.52, and rhombohedral in the range of 0.52<x≦1. The maximum axis ratio a/c of the tetragonal system is 1.07. Each axis of rhombohedral crystal is equal and an angle between axes is 89.9 to 90 degrees. Therefore, these tetragonal crystal and rhombohedral crystal are approximately cubic crystal, and will be described by approximating them to cubic crystal in the following.
The polarization axis of tetragonal PZT is <0 0 1>. The polarization amounts obtained by (1 1 1) and (0 0 1) orientated tetragonal PZT are shown in FIGS. 5B and 5C.
FIG. 5B shows the state of polarization in a (1 1 1) orientated PZT film. Since the polarization axis is <0 0 1>, polarization occurs in three <0 0 1> directions equivalent relative to a plane normal <1 1 1>. A contribution of polarization in the <0 0 1> direction upon the <1 1 1> direction is nearly cos(54.7°)=0.58.
FIG. 5C shows the state of polarization in a (0 0 1) orientated PZT film. It is assumed that polarization occurs equivalently in three <0 0 1> directions. Polarization in one <0 0 1> direction occurs along a plane normal <0 0 1> so that the contribution of this polarization is 1. Polarizations in two other directions, i.e. along <1 0 0> and <0 1 0> directions are 0. An average contribution of the polarization in the direction <0 0 1> is therefore ⅓.
The effective contribution of the polarization of a (1 1 1) orientated film is 0.58 which is larger than the effective contribution of ⅓ of a (0 0 1) orientated film. Therefore, the (1 1 1) orientated film is used generally and the (0 0 1) orientated film is not. In order to realize the (1 1 1) orientation, expensive noble metal such as platinum is used as the material of a lower capacitor electrode.